Hydrogeologic Framework

This section describes the hydrogeologic framework,
which defines the physical, lithologic, and hydrologic characteristics of the
hydrogeologic units that compose the ground-water system in the individual basins.
An understanding of these characteristics is important in determining the occurrence
and availability of ground water within each basin. The ground-water flow system
in each sedimentary basin is part of the larger Yakima River Basin aquifer system.
In turn, the part of the aquifer system contained by the CRBG is part of the
Columbia Plateau regional aquifer system.

The hydrogeologic units vary by basin, and will be
discussed separately for each basin. The hydrogeologic units identified in this
report do not necessarily correspond to geologic time-stratigraphic deposits.
The ground-water flow system within the basin-fill deposits in each basin is
generally isolated from other basins but is interconnected with the larger Yakima
River Basin aquifer system.

Knowledge of the geologic structure that exists within
and surrounds each basin (fig. 7) also assisted in
mapping of the hydrogeologic units. The structural setting helps to explain
the depositional sequences, thickness variations, and segmentation of the ground-water
movement or anomalous water-level distributions within the area. For example,
a predominantly fine-grained unit could be vertically offset and juxtaposed
with a coarse-grained unit, thereby truncating the lateral ground-water movement
along a fault and also offsetting the water-level distribution on either side
of the fault. Although these types of questions are not addressed in this report,
the structural history has provided information for mapping the hydrogeologic
units and should provide additional information related to the ground-water
flow system.

Roslyn Basin

The Roslyn Basin is located southeast of the Kachess
and Cle Elum Lakes (figs. 6and 7)
and encompasses an area of about 80 mi2. The south-central part of
the basin is dissected by two northwest trending faults with their down-thrown
sides toward the northeast, and the northeast part of the basin is divided by
a series of synclines and anticlines (pl.
1).

The basin-fill deposits consist predominantly of alluvial,
lacustrine, and glacial deposits interspersed with Mesozoic metamorphics and
Tertiary volcanic deposits. The deposits were divided and mapped into three
hydrogeologic units—Units 1 through 3 and a total basin thickness. Information
from 307 well logs was used to define the units in the Roslyn Basin. This basin
has been the least studied for ground-water availability, thus, only 31 percent
of the well logs used to identify the hydrogeologic units and basin thickness
were field located and most of these were inventoried during this study.

The three hydrogeologic units consist of an upper coarse-grained
unit, a clay unit, and a productive gravel unit (pl.
1). Unit 1 consists of the alluvial, lacustrine, and glacial deposits at
and near the surface. The thickness of the unit ranges from 0 to 360 ft, with
a mean and median thickness of 80 ft. The altitude of this unit ranges from
1,760 to 3,420 ft, with a mean and median altitude of 2,120 and 2,130 ft, respectively.
Unit 2 consists predominantly of fine-grained, lacustrine deposits of clay and
silt. The extent and thickness of the unit tends to follow the alluvium valley
(pl.
1). The thickness of this unit ranges from 0 to 530 ft with a mean and median
thickness of 180 and 170 ft, respectively. The altitude of this unit ranges
from 1,720 to 2,100 ft, with a mean and median altitude of 1,910 and 1,890 ft,
respectively. Unit 3 consists of coarser deposits, mostly sand and gravels.
It is less extensive than the other units and occurs in the deeper parts of
the alluvial valley (pl.
1). The thickness of Unit 3 ranges from 0 to 240 ft with a mean and median
thickness of 60 and 50 ft, respectively. The altitude of this unit ranges from
1,330 to 2,000 ft with a mean and median altitude of 1,670 and 1,690 ft, respectively.

The thickness of the basin-fill deposits in the Roslyn
Basin is greatest in the central part of the basin in the Yakima alluvial valley
(pl.
1). The total thickness of the basin-fill deposits in the Roslyn Basin ranges
from 0 to 700 ft with a mean and median thickness of 150 and 110 ft, respectively.
The altitude of the top of the bedrock units in this basin ranges from 1,190
to 3,330 ft, with a mean and median altitude of 1,990 and 2,010 ft, respectively.

Kittitas Basin

The Kittitas Basin lies within the northeastern part
of the Yakima River Basin and encompasses an area of about 270 mi2.
The basin is bordered on the southwest by the Ainsley Canyon and Manastash Anticlines,
on the northeast by the Wilson Creek Monocline, and is bisected by the Kittitas
Valley Syncline (fig. 7). The northeastern part of
the basin also contains several east-west and northwest trending high-angle
faults with the downthrown side generally to the north and northeast. The southwestern
part of the basin contains and is bordered by northwest trending strike-slip
and thrust faults (pl.
2).

The basin-fill deposits consist predominantly of alluvial,
loess, glacial, continental sedimentary, and Ellensburg Formation deposits.
The deposits were divided and mapped into three hydrogeologic units—Units 1
through 3 and a total basin thickness. Information from 419 well logs was used
to define the units of the Kittitas Basin. Sixty percent of these wells were
field located, and 40 percent were not field located but supplied additional
information in areas with limited data.

The three hydrogeologic units consist of alluvial,
unconsolidated, and consolidated deposits. Unit 1 consists of the alluvial deposits
and the delineation of this unit was confined to the Yakima River floodplain
(pl. 2). The unit thickness ranges
from 0 to 100 ft, with a mean and median thickness of 30 and 10 ft, respectively.
The altitude of this unit ranges from 1,410 to 1,800 ft, with a mean and median
altitude of 1,530 and 1,520 ft, respectively. Unit 2 consists of the unconsolidated
deposits, and generally contains the loess, alluvial fan, glacial, terrace,
and Thorp gravel deposits present at the surface and beneath Unit 1 (pl.
2). The thickness of this unit ranges from 0 to 790 ft, with a mean and
median thickness of 180 and 150 ft, respectively. The altitude of this unit
ranges from 1,370 to 3,890 ft, with a mean and median altitude of 1,930 and
1,870 ft, respectively. Unit 3 is present along the syncline that parallels
the southwest boundary of the basin (pl.
2). The consolidated deposits of this unit consist of the Ellensburg Formation
and similar undefined continental sedimentary deposits. The thickness of Unit
3 ranges from 0 to 2,040 ft, with a mean and median thickness of 600 and 350
ft, respectively. The altitude of this unit ranges from 770 to 2,700 ft, with
a mean and median altitude of 1,540 and 1,520 ft, respectively.

The thickness of the basin-fill deposits in the Kittitas
Basin is greatest in the south-central part of the basin generally along the
Kittitas Valley Syncline (fig. 7 and pl.
2). The total thickness of the basin-fill deposits in the Kittitas Basin
ranges from 0 to 2,120 ft, with a mean and median thickness of 500 and 270 ft,
respectively (pl.
2). The altitude of the top of the CRBG within the Kittitas Basin ranges
from -540 to 3,610 ft, with a mean and median altitude of 780 and 1,600 ft,
respectively.

Selah Basin

The Selah Basin, also referred to as the Selah-Wenas
Basin, is located in the central part of the Yakima River Basin and encompasses
about 170 mi2. The basin is bounded on the northeast by the Manastash
and Umtanum Anticlines and bounded on the southwest by the Yakima Ridge structure
(figs. 6 and 7). The central
part of this basin is dissected by a series of northwest-southeast trending
folds (fig. 7).

The basin-fill deposits consist predominantly of alluvial,
alluvial fan, loess, terrace, continental sedimentary, and Ellensburg Formation
deposits. The deposits were divided and mapped into three hydrogeologic units—Units
1 through 3 and a total basin thickness (pl.
3). Information from 720 well logs was used to define the units in the Selah
Basin. Eighty-nine percent of these wells were field located and 56 percent
of the field located wells were located in previous investigations.

The three hydrogeologic units consist of the alluvial,
unconsolidated, and consolidated deposits. Unit 1 consists of the alluvial deposits
that are contained in the floodplains of Wenas and Cowiche Creeks and the Naches
and Yakima Rivers (pl. 3). The thickness
of Unit 1 ranges from 0 to 90 ft, with a mean and median thickness of 30 ft.
The altitude of Unit 1 ranges from 1,080 to 2,450 ft, with a mean and median
altitude of 1,470 and 1,420 ft, respectively. Unit 2 consists of the unconsolidated
deposits, and generally contains the alluvial fan, loess, terrace, and Thorp
gravel deposits present as a thin veneer over much of the surface and beneath
Unit 1 (pl. 3). The thickness of this
unit ranges from 0 to 290 ft, with a mean and median thickness of 50 and 40
ft, respectively. The altitude of this unit ranges from 1,050 to 2,760 ft, with
a mean and median altitude of 1,660 and 1,630 ft, respectively. Unit 3 is present
throughout most of the basin (pl. 3)
and consists of the consolidated deposits, which include the Ellensburg Formation
and similar undefined continental sedimentary deposits. The thickness of Unit
3 ranges from 0 to 1,920 ft, with a mean and median thickness of 320 and 200
ft, respectively. The altitude of the unit ranges from 980 to 3,160 ft, with
a mean and median altitude of 1,740 and 1,690 ft, respectively.

The thickness of the basin-fill deposits in the Selah
Basin is greatest in the north-central part of the basin (pl.
3). The thickness of the deposits ranges from 0 to 1,920 ft, with a mean
and median thickness of 300 and 200 ft, respectively. The altitude of the top
of the CRBG in the Selah Basin ranges from -300 to 3,100 ft, with a mean and
median altitude of 1,460 and 1,470 ft, respectively.

Yakima Basin

The Yakima Basin, also referred to as the Ahtanum-Moxee
Basin, is a long narrow east-west trending basin in the central part of the
Yakima River Basin and encompasses an area of about 230 mi2. The
basin is bounded on the north by the Yakima Ridge, on the south by the Ahtanum
Ridge, and is bisected by the Ahtanum-Moxee Syncline (figs.
6 and 7).

The basin-fill deposits consist predominantly of alluvial,
alluvial fan, loess, continental sedimentary, and Ellensburg Formation deposits.
The deposits were divided and mapped into three hydrogeologic units—Units 1
through 3 and a total basin thickness. Information from 995 well logs was used
to define the units in the Yakima Basin. Ninety-nine percent of the wells were
field located and 51 percent of these wells were located in previous investigations.

The three hydrogeologic units consist of the alluvial,
unconsolidated, and consolidated deposits. Unit 1 consists of the alluvial deposits
that are contained in the floodplains of Ahtanum Creek and the Yakima River
(pl. 4). The thickness of Unit 1 ranges
from 0 to 120 ft, with a mean and median thickness of 20 ft. The altitude of
this unit ranges from 940 to 2,780 ft, with a mean and median altitude of 1,270
and 1,180 ft, respectively. Unit 2 consists of the unconsolidated deposits,
which include the alluvial fan, loess, terrace, and Thorp gravel deposits. Unit
2 generally is present at the surface and beneath Unit 1 (pl.
4). The thickness of Unit 2 ranges from 0 to 350 ft, with a mean and median
thickness of 90 and 80 ft, respectively. The altitude of this unit ranges from
910 to 2,940 ft, with a mean and median altitude of 1,480 and 1,460 ft, respectively.
Unit 3 is present throughout most of the basin with the general exception of
its western tip (pl. 4). Unit 3 consists
of the consolidated deposits, principally the Ellensburg Formation and similar
undefined continental sedimentary deposits. The thickness of Unit 3 ranges from
0 to 1,840 ft with a mean and median thickness of 510 and 450 ft, respectively.
The altitude of this unit ranges from 760 to 2,780 ft, with a mean and median
altitude of 1,330 and 1,320 ft, respectively.

The thickness of the basin-fill deposits in the Yakima
Basin is greatest in the northwestern part of the basin (pl.
4). Its thickness ranges from 0 to 1,840 ft, with a mean and median thickness
of 530 and 410 ft, respectively. The altitude of the top of the CRBG in the
Yakima Basin ranges from -510 to 2,900 ft, with a mean and median altitude of
980 and 1,840 ft, respectively.

Toppenish Basin

The Toppenish Basin lies in the south-central part
of the Yakima River Basin and encompasses an area of about 440 mi2.
The basin is bordered on the north by the Ahtanum Ridge, on the south by the
Toppenish Ridge, and bisected by the Wapato Syncline (fig.
7). The eastern boundary of this basin abuts the Benton Basin (figs.
6 and 7).

The basin-fill deposits consist predominantly of alluvial,
alluvial fan, loess, terrace, continental sedimentary, Touchet, and Ellensburg
Formation deposits. The deposits in this basin were divided and mapped into
hydrogeologic units—Units 1 through 5 and a total basin thickness. The hydrogeologic
unit delineations were modified from contour maps constructed by Campbell (unpub.
maps produced for Yakama Nation, 2001).To derive the modified contour maps,
a total basin thickness map was constructed first based on information from
1,104 well logs of which 210 well logs fully penetrated the basin thickness
(pl.
5). Ninety-seven percent of the well logs used were field located. The thickness
map was then subtracted from the DEM to obtain a map of the top of the CRBG.
This map was then compared to the top of the CRBG map compiled by Campbell (unpub.
maps produced for Yakama Nation, 2001). The maps were a close match in both
the shape of the contours and the contour values. Small differences were predominantly
due to newer maps of the surficial geology and the extent that was based on
our criteria that the depth of the basin-fill deposits was defined as contact
with the first basalt. The contour maps for the units were then digitized and
modified based on the updated surficial geology and on the occurrence of the
first basalt contact.

The five hydrogeologic units consist of fine-grained
and coarse-grained unconsolidated deposits, consolidated deposits, top of the
Rattlesnake Ridge unit, and base of the Rattlesnake Ridge unit. Unit 1 consists
of fine-grained unconsolidated surficial deposits that include Touchet, terrace,
loess, and some alluvial deposits (pl.
5). The thickness of Unit 1 ranges from 0 to 80 ft, with a mean and median
thickness of 10 ft. The altitude of this unit ranges from 690 to 2,030 ft, with
a mean and median altitude of 910 and 860 ft, respectively. Unit 2 consists
predominantly of coarse-grained unconsolidated deposits. The thickness of Unit
2 ranges from 0 to 270 ft, with a mean and median thickness of 90 and 80 ft,
respectively. The altitude of this unit ranges from 660 to 2,310 ft, with a
mean and median altitude of 930, and 850 ft, respectively. Unit 3 is present
throughout most of the basin (pl. 5).
It consists predominantly of the consolidated deposits of the upper Ellensburg
Formation and similar undefined continental sedimentary deposits. The thickness
of Unit 3 ranges from 0 to 970 ft, with a mean and median thickness of 350 and
320 ft, respectively. The altitude of this unit ranges from 510 to 2,300 ft,
with a mean and median altitude of 900 and 820 ft, respectively. Unit 4 is present
in the central part of the basin (pl. 5),
and consists of fine-grained deposits. This unit is referred to as the top of
the Rattlesnake Ridge unit of the Ellensburg Formation and the “Blue Clay” layer
by Campbell (unpub. maps produced for Yakama Nation, 2001). The thickness of
Unit 4 ranges from 0 to 520 ft, with a mean and median thickness of 170 and
140 ft, respectively. The altitude of this unit ranges from -350 to 1,130 ft,
with a mean and median altitude of 320 and 330 ft, respectively. Unit 5 which
has an extent similar to Unit 4 (pl. 5)
consists predominantly of coarse-grained gravels. This unit is referred to as
the base of Rattlesnake Ridge unit of the Ellensburg Formation by Campbell (Tom
Ring, Yakama Nation, written commun., 2001). The thickness of Unit 5 ranges
from 0 to 140 ft, with a mean and median thickness of 20 ft. The altitude of
this unit ranges from -400 to 1,020 ft, with a mean and median altitude of 130
ft.

The thickness of the basin-fill deposits in the Toppenish
Basin is greatest in the central-southeastern part of the basin (pl.
5). The thickness of the basin-fill deposits ranges from 0 to 1,210 ft,
with a mean and median thickness of 550 ft. The altitude of the top of the CRBG
in the Toppenish Basin ranges from -410 to 2,290 ft, with a mean and median
altitude of 450 and 380 ft, respectively.

Benton Basin

The Benton Basin lies in the southeastern part of the
Yakima River Basin. It is the largest of the six basins encompassing an area
of about 1,020 mi2. The western boundary of the Benton Basin abuts
the eastern boundary of the Toppenish Basin and a small section of the Yakima
Basin (figs. 6 and 7).
The southern boundary is bordered by the Horse Heaven Hills structure and the
northeastern boundary generally follows the northern flank of the Cold Creek
Syncline (figs. 6 and 7).
The basin is dissected with numerous faults and folds surrounding the Rattlesnake
Hills structure in the eastern part of the basin (fig.
7 and pl.
6). The western part of the basin is dissected by the Wapato Syncline and
several unnamed folds that lie within the broad flat plain that encompasses
the Yakima River floodplain (fig. 7 and pl.
6). Sediment deposits are thickest on the northeastern side of the Rattlesnake
Hills structure, which separates the sediment deposits on the northeastern side
from those on the southwestern side.

The basin-fill deposits consist of loess, alluvial
fan, dune sand, alluvial, terrace, continental sedimentary, and Ellensburg Formation
deposits. The deposits for the entire Benton Basin were divided and mapped into
two hydrogeologic units— Units 1 and 2 and a total basin thickness. Information
from 1,141 well logs was used to define the units of the Benton Basin. All of
these wells were field located. Sixty-five percent of the wells were field located
during this study and 35 percent were located in previous investigations.

The two hydrogeologic units consist of unconsolidated
and consolidated deposits. Unit 1 consists of the unconsolidated deposits that
generally consist of the alluvial, alluvial fan, loess, terrace, dune sand,
Touchet, Missoula flood and Ringold Formation deposits (pl. 6, sheet
1 and sheet 2). The thickness
of Unit 1 ranges from 0 to 870 ft, with a mean and median thickness of 120 and
70 ft, respectively. The altitude of this unit ranges from 300 to 3,620 ft,
with a mean and median altitude of 990 and 830 ft, respectively. The unconsolidated
basin-fill deposits generally are less than 400 ft throughout most of the basin
except in the area northeast of the Rattlesnake Hills structure, where thicknesses
exceed 800 ft (pl. 6, sheet 2).

Additional information incorporated from previous studies
(Drost and others, 1997; Reidel and Thorn, Battelle, written commun., 2003 and
2005) enabled the unconsolidated unit in the east-northeast area of the basin
to be divided into six subunits (figs. 8A
through 8F). Subunit 1 consists of the fine-grained
Touchet and similar deposits present at the surface. The thickness of subunit
1 ranges from 0 to about 300 ft (fig. 8A).
Subunit 2 consists of the coarse-grained Pasco gravels and similar deposits
present at the surface and beneath subunit 1. The thickness of subunit 2 ranges
from 0 to about 100 ft (fig. 8B). Subunit
3 is of limited extent and consists of the fine-grained deposits of the upper
Ringold Formation present beneath Subunit 2. The thickness of subunit 3 ranges
from 0 to about 50 ft (fig. 8C). Subunit
4, present throughout most of the northeastern area of the Benton Basin, consists
of the coarse-grained deposits of the middle Ringold Formation. The thickness
of subunit 4 ranges from 0 to about 400 ft (fig. 8D).
Subunit 5, also present throughout most of the northeastern area of the Benton
Basin, consists of the fine-grained deposits of the lower Ringold Formation.
The thickness of subunit 5 ranges from 0 to about 250 ft (fig.
8E). Subunit 6 generally is present beneath Subunit 5, and consists
of the coarse-grained basal Ringold Formation. The thickness of Subunit 6 ranges
from 0 to about 200 ft (fig. 8F).

Unit 2 is mainly present in the southwestern part of
the Benton Basin (pl.
6, sheet 2). The consolidated deposits of this unit include the Ellensburg
Formation and similar undefined continental sedimentary deposits. The thickness
of Unit 2 ranges from 0 to 680 ft, with a mean and median thickness of 100 and
60 ft, respectively. The altitude of this unit ranges from 370 to 3,400 ft,
with a mean and median altitude of 890 and 790, respectively.

The basin-fill deposits of the Benton Basin are thickest
to the west where the basin abuts the Toppenish Basin and in the northeastern
area of the basin (pl. 6, sheet 2).
The thickness of the basin-fill deposits in the Benton Basin ranges from 0 to
870 ft, with a mean and median thickness of 120 and 60 ft, respectively. The
altitude of the top of the CRBG in the Benton Basin ranges from -300 to 3,630
ft, with a mean and median altitude of 960 and 800 ft, respectively.